Show error message for unexpected errors

[andresmgot]

Fixes #856

Shows the cause of the error for the UnexpectedError page.

Additional change: To ensure we don't show the generic message by mistake in some view, now it's explicitly necessary to set its flag (showGenericMessage ) to true to render it.

[migmartri]

Is showGenericMessage={false} ever used? If not, I'd remove the feature and always show the GenericMessage alongwith the raw error.

[prydonius]

I believe showGenericMessage={false} is used (set by defaultProps) when we don't only want to show the raw error message.

[andresmgot]

Yes, it's used in several places like in the PermissionErrorAlert. It's not necessary now because the default is false now but I think it's useful to be explicit.